The present invention relates to a circuit configuration for controlling the braking pressure in the wheel brakes of a slip-controlled hydraulic brake system for automotive vehicles with all-wheel drive and with lockable or automatically locking differentials. Sensors are provided for measuring the wheel rotational behavior as are electronic circuits for logically combining and processing the sensor signals and for generating braking-pressure control signals. By means of the signals, the braking pressure is variable as a function of the wheel rotational behavior and a vehicle reference speed. The reference speed is derived from the sensor signals and which serves as a reference value for proportioning the braking pressure in the individual wheel brakes in response to the rotational behavior of the respective wheel. The reference speed, according to predefined criteria, is temporarily determined by the speed of a specific wheel, for example, the fastest or the second fastest wheel, and temporarily rising or decling with a specific gradient.
Electronic circuits for controlling the valves, by virtue of which the braking pressure in the wheel brakes of slip-controlled hydraulic brake systems is modulated, are known. To control the wheel slip, the rotational behavior of the wheels is measured and compared with a reference value, frequently termed as vehicle reference speed. From the discrepancy of the actual rotational behavior of a wheel from the reference value, which could include taking into consideration further criteria and measured values in respect of driving dynamics, conclusions can be drawn in regard to the control and variation of the pressure which is required for the adjustment of an optimal wheel slip and thus for ensuring a short stopping distance, while driving stability and steerability are also preserved.
In conventional practice, the information relating to the actual vehicle speed and vehicle deceleration, wherein, in turn, the vehicle reference speed can be derived as a reference variable for controlling the braking pressure is gained with the aid of wheel sensors and by logic combining and processing of the measured signals dependent on the rotational behavior of the individual wheels.
The vehicle reference speed can be interpreted as vehicle speed at optimal brake slip. As the reference speed serves as a reference variable for controlling the braking pressure, it is of decisive importance for the attainable quality of slip control. While in vehicles with only one driven axle the free-running non-driven wheels and their comparison to the driven wheels allow precise conclusions with respect to the speed and deceleration of the vehicle, the determination of an appopriate vehicle reference speed in vehicles with all-wheel drive entails major difficulties which are further complicated by the presence of locks in the differentials. The coupling by way of the drive shafts, the differentials and differential locks in many situations has as a result a largely synchronous wheel rotational behavior which makes it very difficult to judge the actual wheel slip and the vehicle behavior. The masses increased by the co-rotating drive shafts and driving wheels, moreover, result in high moments of inertia which delay the reaction on braking pressure variations. The transfer of moments by way of the coupled driving results in an apparent increase of the moments of inertia.
In order to be able to use slip-controlled brake system in vehicles with all-wheel drive and with differential locks desptie these difficulties, vehicles have been known wherein the engagement of the differential locks is coincident with a disconnection of slip control. It is, however, a disadvantage that especially under such road conditions where slip control would otherwise provide for driving stability and steerability of the vehicle, such control must be dispensed with.
Another suggestion to resolve the problems resides in that the differential lock is automatically released upon commencement of slip control (German patent application No. 34 18 520.8). It has likewise been proposed to insert a disconnecting clutch into the split-axle transferring the driving torque form the differential onto the wheel, which disconnecting clutch will automatically disengage upon commencement of slip control (German patent application No. 34 26 747. 6). Both suggestions precondition to mount clutches actuatable by the slip control for the purpose of interruption of the driving tracks or for release of the differential locks.
It is, therefore, an object of the present invention to overcome these shortcomings and to provide a circuit configuration which, despite the transmission of torques via the driving tracks and despite the actual as well as the seemingly high moments of inertia, permits control of the brake slip of an all-wheel driven automotive vehicle equipped with lockable differentials and to thereby safeguard a short stopping distance and high driving stability at the same time.